NERVE-DEPENDENT FACTORS REGULATING TRANSCRIPT LEVELS OF GLYCOGEN-PHOSPHORYLASE IN SKELETAL-MUSCLE

1. Muscle glycogen phosphorylase (MGP), the rate-limiting enzyme for g lycogen metabolism in skeletal muscle, is neurally regulated. Steady-s tate transcript levels of the skeletal muscle isozyme of MGP decrease significantly following muscle denervation and after prolonged muscle inactivity with an intact motor nerve. These data suggest that muscle activity has an important influence on MGP gene expression. The eviden ce to this point, however, does not preclude the possibility that MGP is also regulated by motor neuron-derived trophic factors. This study attempts to distinguish between regulation provided by nerve-evoked mu scle contractile activity and that provided by the delivery of neurotr ophic factors. 2. Steady-state MGP transcript levels were determined i n rat tibialis anterior (TA) muscles following controlled intervention s aimed at separating the contributions of contractile activity from a xonally transported trophic factors. The innervated TA was rendered in active by daily epineural injections of tetrodotoxin (TTS) into the sc iatic nerve. Sustained inhibition of axonal transport was accomplished by applying one of three different concentrations of the antimicrotub ule agent, vinblastine (VIN), to the proximal sciatic nerve for 1 hr. The axonal transport of acetylcholinesterase (AChE) was assessed 7, 14 , and 28 days after the single application of VIN. 3. MGP transcript l evels normalized to total RNA were reduced by 67% in rat TA, 7 days af ter nerve section. Daily injection of 2 mu g TTX into the sciatic nerv e for 7 days eliminated muscle contractile activity and reduced MGP tr anscript levels by 60%. 4. A single, 1-hr application of 0.10% (w/v) V IN to the sciatic nerve reduced axonal transport but did not alter MGP transcript levels in the associated TA, 7 days after treatment. Appli cation of 0.10% VIN to the sciatic nerve also did not affect IA sensor y or motor nerve conduction velocities or TA contractile function. 5. Treatment of the sciatic nerve with 0.40% (w/v) VIN for 1 hr reduced a xonal transport and decreased MGP transcript levels by 50% within 7 da ys, but also reduced sensory and motor nerve conduction velocities and depressed TA contractile function. 6. Myogenin, a member of a family of regulatory factors shown to influence the transcription of many mus cle genes, including MGP, was used as a molecular marker for muscle in activity. Myogenin transcript levels were increased following denervat ion and after treatment with TTX or 0.40% VIN but not after treatment with 0.10% VIN. 7. The results suggest that MGP transcript levels in T A are regulated predominantly by muscle activity, rather than by the d elivery of neurotrophic factors. Intrinsic myogenic factors, however, also play a role in MGP expression, since denervation did not reduce M GP transcript levels below 30% of control TA. The dominant influence o f activity in the regulation of MGP contrasts with the proposed regula tion of oxidative enzyme expression, which appears to depend on both a ctivity and trophic factor influences.